Error correction code for unidirectional memory

ABSTRACT

A memory array and a method of writing to a unidirectional non-volatile storage cell are disclosed whereby a user data word is transformed to an internal data word and written to one or more unidirectional data storage cells according to a cell coding scheme. A check word may be generated that corresponds to the internal data word. In some embodiments, the check word may be generated by inverting one or more bits of an intermediate check word. Other embodiments may be described and claimed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a direct national filing claiming priority toInternational Patent Application No. PCT/IT08/00824, filed Dec. 31,2008, entitled “Error Correction Code for Unidirectional Memory,” thespecification of which is incorporated by reference herein.

FIELD

The present disclosure is generally directed to memory and moreparticularly to method and apparatus for applying Error Correction Code(ECC) in a unidirectional memory.

BACKGROUND

Memory is an integral part of many electronic devices for storing andretrieving information. Memory may be classified on different bases,such as amount of data (for example, mass storage versus cache memory),access (for example, random access memory versus read-only memory),technology (for example, magnetic versus optical), performance (forexample, archival versus execution memory), and the like. With theubiquity of computers and communication networks for personal andbusiness use, the need for data storage has increased exponentially inrecent years. This need is especially increased due to the abundance andconstant production of multimedia data such as music, video clips, andpictures. Commensurate with increased use of memory is the increasedneed for reliability of storage medium and also need for storage of datain absence of power. Relatively recent technologies used for solid-statestorage of data, especially for mobile devices, include flash memoriesand phase-change memories (PCM) that retain stored information inabsence of power. To increase reliability of data and reduce storage andtransmission errors, error detection and/or correction codes, such asparity bits and ECC, may be used. With the non-volatile, solid-statememory technologies that depend on unidirectional programming, an updateof data, such as a second write, is not possible without losingcoherence of ECC check bits, unless, the memory is first erased and newECC check bits are calculated. The process of erasure takes time andreduces memory and/or system performance.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present disclosure are described withreference to the following drawings. For a better understanding of thepresent disclosure, the detailed description that follows is to be readin association with the accompanying drawings referenced therein,wherein:

FIG. 1 shows an embodiment of a network computing environment whereinthe disclosure may be practiced;

FIG. 2 shows an embodiment of a computing device that may be used in thenetwork computing environment of FIG. 1;

FIG. 3 shows an embodiment of a memory array including a plurality ofmemory cells;

FIG. 4 shows an embodiment of a flash memory cell;

FIG. 5 shows a graph depicting an illustrative example of data and ECCcheck bits;

FIG. 6 shows an illustrative cell coding scheme;

FIG. 7 shows an illustrative matrix A used in calculation of ECC;

FIG. 8 shows an illustrative application of matrix A to user data;

FIG. 9 shows an illustrative application of internal level coding touser data; and

FIG. 10 shows an illustrative process of protecting user data with ECCcheck bits according to aspects of present disclosure.

DETAILED DESCRIPTIONS

The present disclosure will now be described more fully hereinafter withreference to the accompanying drawings, which form a part hereof, andwhich show, by way of illustration, specific embodiments of the presentdisclosure. This disclosure may be embodied in many different forms andshould not be construed as limited to the embodiments set forth herein.Rather, these embodiments are provided so that this disclosure isthorough and complete, and may fully convey the scope of the disclosureto those of ordinary skill in the art. Among other things, the presentdisclosure may be embodied as methods or devices. Accordingly, thepresent disclosure may be in the form of an entirely hardwareembodiment, an entirely software embodiment or an embodiment combiningsoftware and hardware aspects. The following detailed description is,therefore, not to be taken in a limiting sense.

Briefly described, in one aspect of the present disclosure an electronicor computing device is disclosed including a processor configured toprocess input data and a plurality of unidirectional data storage cellscoupled with the processor. Each of the data storage cells have a numberof ordered states for storing at least a portion of the input data. Thedata storage cells are configured to represent data according to aninternal coding. The data storage cells perform a first write operationto change the first data to second data; perform a second writeoperation to change the second data to third data; and generate acoherent check word based on the third data without performing an eraseoperation on the data storage cell. Customarily, because of the natureof a unidirectional memory, an erase operation is required to reset thestorage cell for new data if the new data requires the cell state tochange in an opposite direction. For example, a capacitive memory withfour states may be charged-up three times, but it typically cannot becharged down from any state to its previous state. In such cases, forexample if the state of the capacitive cell is to be changed from state#3 to state #2, the cell capacitor typically needs to be completelydischarged and again be charged-up to state #2.

In another aspect of the present disclosure a memory array having aplurality of unidirectional data storage cells is disclosed, some ofwhich have a plurality of ordered states and are configured to use amemory interface to access the data storage cells. The data storagecells represent first data according to an internal coding; perform afirst write operation to change the first data to second data; perform asecond write operation to change the second data to third data; andgenerate a coherent check word based on the third data withoutperforming an erase operation on the data storage cell.

In yet another aspect of the present disclosure a memory array having aplurality of unidirectional data storage cells is disclosed, some ofwhich have a plurality of ordered states, and are configured to use amemory interface to access data storage cells using a multi-componentaddress. The data storage cells represent first data according to aninternal coding; generate a first check word at least partially based onthe first data; and invert at least one bit in the first check word togenerate a first coherent check word associated with the first data.

In yet another aspect of the present disclosure a method of writing datato a unidirectional memory device is disclosed. The method includesrepresenting first data according to an internal coding; performing afirst write operation to change the first data to second data;performing a second write operation to change the second data to thirddata; and generating a coherent check word at least partially based onthe third data without performing an erase operation on theunidirectional memory device.

Throughout the specification and claims, the following terms take themeanings explicitly associated herein, unless the context clearlydictates otherwise. The phrases “in one embodiment,” “in an illustrativeembodiment,” and the like, as used herein do not necessarily refer tothe same embodiment, though they may. As used herein, the term “or” isan inclusive “or” operator, and is equivalent to the term “and/or,”unless the context clearly dictates otherwise. The term “based on” isnot exclusive and allows for being based on additional factors notdescribed, unless the context clearly dictates otherwise. In addition,throughout the specification, the meaning of “a,” “an,” and “the”include plural references. The meaning of “in” may include “in” and“on,” unless otherwise stated.

The term “memory array” as used herein, may represent a logical grid(which may or may not correspond to a physical grid) of memorycomponents that are addressable by two address components, such as a rowvalue and a column value. Additionally, the term memory array may referto a multi-dimensional or N-dimensional array, addressable by Nindependent address components, such as 3-dimensional arrays or banks of2- or 3-dimensional arrays.

The terms “flash,” “flash memory,” “NOR flash,” “NAND flash,” and thelike, as used herein, may indicate non-volatile memories that may beused to store discrete data, binary or otherwise, in a unidirectionalmanner, such that internal states of the memory may be changed fromstate [i] to state [i+1], but not from state [i+1] back to state [i]without first erasing the memory to reset the memory to an initialstate.

The term “cell” or “storage cell,” as used herein, may denote a storagecomponent of the memory array where actual data are recorded andrepresented. The storage cell may have multiple internal states orlevels that may be used to represent binary data. In some memorytechnologies, such as Phase-Change-Memory (PCM) cells, the storage cellmay be multi-phase, where each phase may correspond to a state. Thesestates may include “Set,” “Reset,” and one or more intermediate states.

The terms “internal state,” “memory state,” and the like, as usedherein, may indicate physically or electrically distinct states ofstorage cell material, such as a crystalline state or a charged state,respectively, used to represent a logical/Boolean value, such as alogic-1, or logic-0. Those skilled in the art will appreciate thatrepresentation of logic-1 and logic-0 values are arbitrary and suchrepresentations may be freely chosen as one symbol or another, as longas such representation is applied consistently throughout a systemand/or process. For example, a “high” signal level or a signaltransition from low-to-high may be chosen to represent logic-0 orlogic-1.

The terms “ECC,” “error correction code,” “error detection andcorrection code,” “check bit,” “parity bit,” “ECC data,” “parity data,”and the like, as used herein, may indicate bits of binary informationcalculated from bits of application and/or user data to detect and/orcorrect corrupted bits in application/user data.

The terms “data,” “user data,” “application data,” “original data,”“input data,” and the like, as used herein, may indicate data that isprovided before application of an error detection/correction scheme,such as ECC.

The terms “codeword,” “code,” and the like, as used herein, may denotethe output data after the application of an error detection/correctionscheme to the input data plus check bits that are generated as a resultof applying the error detection/correction to the input data, as oneunit. The portion of the codeword representing the input data, asdistinct from check bits, may be encoded or may be identical to theinput data.

The terms “level,” “flash level,” “flash cell level,” and the like, asused herein, may indicate distinct internal (usually unidirectional)states of a storage cell, where the levels are arranged as an orderedsequence and associated with binary data.

The terms “coding,” “data coding,” “internal coding,” “encoding,” dataencoding,” “level coding,” “level encoding,” “cell coding,” “cell binarycoding,” and the like, as used herein, may indicate a mapping from or anassignment/association of a binary number to each level included in asequence of levels of a storage cell. For example, level-0 may be mappedto binary number “11,” while level-1 may be mapped to binary number“10.” The binary numbers used in such mapping need not be in numericalorder.

The terms “erasure state,” “initial state,” “level 0 state,” and thelike, as used herein, may indicate the first state that may beestablished in the sequence of levels in a unidirectional storage cell,for example, by storing minimum allowable and/or usable electricalcharge in a storage cell. Alternatively, the first state may beassociated with maximum allowable/usable charge instead of minimum.

The term “final state,” as used herein, may indicate the last state thatmay be established in the sequence of levels in a unidirectional storagecell, for example, by storing maximum allowable and/or usable electricalcharge in a storage cell. Alternatively, the first state may beassociated with minimum allowable/usable charge instead of maximum.

The term “bit inversion,” as used herein, may indicate a subset ofparity check bits that need to be inverted to recover user data afterapplication of an internal coding (level coding) to user data.

The term “transformation,” as used herein, may indicate a change ofrepresentation of user data from a user coding to an internal coding.The term “inverse transformation” may indicate a change ofrepresentation of user data from the internal coding back to the usercoding.

The term “generator matrix,” “generator,” “ECC generator,” “codegeneration matrix,” and the like, as used herein, may indicate a matrixthat may be used to generate ECC bits from user data.

The following briefly describes illustrative embodiments of thedisclosure to provide a basic understanding of some aspects of thedisclosure. This brief description is not intended as an extensiveoverview. It is not intended to identify key or critical elements, or todelineate or otherwise narrow the scope. Its purpose is merely topresent some concepts in a simplified form as a prelude to the moredetailed description that is presented later.

Illustrative Operating Environment

FIG. 1 shows components of an illustrative environment in which thedisclosure may be practiced. Not all the shown components may berequired to practice the disclosure, and variations in the arrangementand type of the components may be made without departing from the spiritor scope of the disclosure. System 100 may include Local Area Networks(LAN) and Wide Area Networks (WAN) shown collectively as Network 106,wireless network 110, gateway 108 configured to connect remote and/ordifferent types of networks together, client devices 112-118, and serverdevices 102-104.

One embodiment of an electronic or computing device usable as one ofclient devices 112-118 is described in more detail below with respect toFIG. 2. Briefly, however, client devices 112-118 may include virtuallyany device capable of receiving and sending a message over a network,such as wireless network 110, or the like. Such devices include portabledevices such as, cellular telephones, smart phones, display pagers,radio frequency (RF) devices, music players, digital cameras, infrared(IR) devices, Personal Digital Assistants (PDAs), handheld computers,laptop computers, wearable computers, tablet computers, integrateddevices combining one or more of the preceding devices, or the like.Client device 112 may include virtually any computing device thattypically connects using a wired communications medium such as personalcomputers, multiprocessor systems, microprocessor-based or programmableconsumer electronics, network PCs, or the like. In one embodiment, oneor more of client devices 112-118 may also be configured to operate overa wired and/or a wireless network.

Client devices 112-118 typically range widely in terms of capabilitiesand features. For example, a cell phone may have a numeric keypad and afew lines of monochrome LCD display on which only text may be displayed.In another example, a web-enabled client device may have a touchsensitive screen, a stylus, and several lines of color LCD display inwhich both text and graphic may be displayed.

A web-enabled client device may include a browser application that isconfigured to receive and to send web pages, web-based messages, or thelike. The browser application may be configured to receive and displaygraphic, text, multimedia, or the like, employing virtually any webbased language, including a wireless application protocol messages(WAP), or the like. In one embodiment, the browser application may beenabled to employ one or more of Handheld Device Markup Language (HDML),Wireless Markup Language (WML), WMLScript, JavaScript, StandardGeneralized Markup Language (SGML), HyperText Markup Language (HTML),eXtensible Markup Language (XML), or the like, to display and sendinformation.

Client devices 112-118 also may include at least one other clientapplication that is configured to receive content from another computingdevice, including, without limit, server computing devices 102-104. Theclient application may include a capability to provide and receivetextual content, multimedia information, or the like. The clientapplication may further provide information that identifies itself,including a type, capability, name, or the like. In one embodiment,client devices 112-118 may uniquely identify themselves through any of avariety of mechanisms, including a phone number, Mobile IdentificationNumber (MIN), an electronic serial number (ESN), mobile deviceidentifier, network address, such as IP (Internet Protocol) address,Media Access Control (MAC) layer identifier, or other identifier. Theidentifier may be provided in a message, or the like, sent to anothercomputing device.

Client devices 112-118 may also be configured to communicate a message,such as through email, Short Message Service (SMS), Multimedia MessageService (MMS), instant messaging (IM), Internet relay chat (IRC),Mardam-Bey's IRC (mIRC), Jabber, or the like, to another computingdevice. However, the present disclosure is not limited to these messageprotocols, and virtually any other message protocol may be employed.

Client devices 112-118 may further be configured to include a clientapplication that enables the user to log into a user account that may bemanaged by another computing device. Such user account, for example, maybe configured to enable the user to receive emails, send/receive IMmessages, SMS messages, access selected web pages, download scripts,applications, or a variety of other content, or perform a variety ofother actions over a network. However, managing of messages or otherwiseaccessing and/or downloading content, may also be performed withoutlogging into the user account. Thus, a user of client devices 112-118may employ any of a variety of client applications to access content,read web pages, receive/send messages, or the like. In one embodiment,for example, the user may employ a browser or other client applicationto access a web page hosted by a web server implemented as servercomputing device 102. In one embodiment, messages received by clientdevices 112-118 may be saved in non-volatile memory, such as flashand/or PCM, across communication sessions and/or between power cycles ofclient devices 112-118.

Wireless network 110 may be configured to couple client devices 114-118to network 106. Wireless network 110 may include any of a variety ofwireless sub-networks that may further overlay stand-alone ad-hocnetworks, and the like, to provide an infrastructure-oriented connectionfor client devices 114-118. Such sub-networks may include mesh networks,Wireless LAN (WLAN) networks, cellular networks, and the like. Wirelessnetwork 110 may further include an autonomous system of terminals,gateways, routers, and the like connected by wireless radio links, andthe like. These connectors may be configured to move freely and randomlyand organize themselves arbitrarily, such that the topology of wirelessnetwork 110 may change rapidly.

Wireless network 110 may further employ a plurality of accesstechnologies including 2nd (2G), 3rd (3G) generation radio access forcellular systems, WLAN, Wireless Router (WR) mesh, and the like. Accesstechnologies such as 2G, 3G, and future access networks may enable widearea coverage for mobile devices, such as client devices 114-118 withvarious degrees of mobility. For example, wireless network 110 mayenable a radio connection through a radio network access such as GlobalSystem for Mobil communication (GSM), General Packet Radio Services(GPRS), Enhanced Data GSM Environment (EDGE), WEDGE, Bluetooth, HighSpeed Downlink Packet Access (HSDPA), Universal MobileTelecommunications System (UMTS), Wi-Fi, Zigbee, Wideband Code DivisionMultiple Access (WCDMA), and the like. In essence, wireless network 110may include virtually any wireless communication mechanism by whichinformation may travel between client devices 112-118 and anothercomputing device, network, and the like.

Network 106 is configured to couple one or more servers depicted in FIG.1 as server devices 102-104 and their respective components with othercomputing devices, such as client device 112, and through wirelessnetwork 110 to client devices 114-118. Network 106 is enabled to employany form of computer readable media for communicating information fromone electronic device to another. Also, network 106 may include theInternet in addition to local area networks (LANs), wide area networks(WANs), direct connections, such as through a universal serial bus (USB)port, other forms of computer-readable media, or any combinationthereof. On an interconnected set of LANs, including those based ondiffering architectures and protocols, a router acts as a link betweenLANs, enabling messages to be sent from one to another.

Communication links within LANs typically include twisted wire pair orcoaxial cable, while communication links between networks may utilizeanalog telephone lines, full or fractional dedicated digital linesincluding T1, T2, T3, and T4, Integrated Services Digital Networks(ISDNs), Digital Subscriber Lines (DSLs), wireless links includingsatellite links, or other communications links known to those skilled inthe art. Furthermore, remote computers and other related electronicdevices could be remotely connected to either LANs or WANs via a modemand temporary telephone link. Network 106 may include any communicationmethod by which information may travel between computing devices.Additionally, communication media typically may enable transmission ofcomputer-readable instructions, data structures, program modules, orother types of content, virtually without limit. By way of example,communication media includes wired media such as twisted pair, coaxialcable, fiber optics, wave guides, and other wired media and wirelessmedia such as acoustic, RF, infrared, and other wireless media.

Illustrative Client Device Configuration

FIG. 2 shows an illustrative electronic or computing device 200 that mayrepresent any one of the server and/or client devices shown in FIG. 1. Aclient device represented by electronic or computing device 200(hereinafter, may be simply “computing device”) may include less or morethan all the components shown in FIG. 2 depending on the functionalityneeded. For example, a mobile client device may include the transceiver236 and antenna 238, while a server device 102 of FIG. 1 may not includethese components. Those skilled in the art will appreciate that thescope of integration of components of computing device 200 may bedifferent from what is shown. As such, some of the components ofcomputing device 200 shown in FIG. 2 may be integrated together as oneunit. For example, NIC 230 and transceiver 236 may be implemented as anintegrated unit. Additionally, different functions of a single componentmay be separated and implemented across several components instead. Forexample, different functions of I/O processor 220 may be separated intotwo or more processing units.

With continued reference to FIG. 2, computing device 200 includesoptical storage 202, Central Processing Unit (CPU) 204, memory module206, display interface 214, audio interface 216, input devices 218,Input/Output (I/O) processor 220, bus 222, non-volatile memory 224,various other interfaces 226-228, network interface, e.g. networkinterface card (NIC) 230, hard disk 232, power supply 234, transceiver236, antenna 238, haptic interface 240, and Global Positioning System(GPS) unit 242. Memory module 206 may include software such as OperatingSystem (OS) 208, and a variety of software application programs 210-212.Computing device 200 may also include other components not shown in FIG.2. For example, computing device 200 may further include an illuminator(for example, a light), graphic interface, and portable storage mediasuch as USB drives. Computing device 200 may also include otherprocessing units, such as a math co-processor, graphicsprocessor/accelerator, and a Digital Signal Processor (DSP).

Optical storage device 202 may include optical drives for using opticalmedia, such as CD (Compact Disc), DVD (Digital Video Disc), and thelike. Optical storage devices 202 may provide inexpensive ways forstoring information for archival and/or distribution purposes.

Central Processing Unit (CPU) 204 may be the main processor for softwareprogram execution in computing device 200. CPU 204 may represent one ormore processing units that obtain software instructions from memorymodule 206 and execute such instructions to carry out computationsand/or transfer data between various sources and destinations of data,such as hard disk 232, I/O processor 220, display interface 214, inputdevices 218, non-volatile memory 224, and the like.

Memory module 206 may include RAM (Random Access Memory), ROM (Read OnlyMemory), and other storage means, mapped to one addressable memoryspace. Memory module 206 illustrates one of many types of computerstorage media for storage of information such as computer readableinstructions, data structures, program modules or other data. Memorymodule 206 may store a basic input/output system (BIOS) for controllinglow-level operation of computing device 200. Memory module 206 may alsostore OS 208 for controlling the general operation of computing device200. It will be appreciated that OS 208 may include a general-purposeoperating system such as a version of UNIX, or LINUX™, or a specializedclient communication operating system such as Windows Mobile™, or theSymbian® operating system. OS 208 may, in turn, include or interfacewith a Java virtual machine (JVM) module that enables control ofhardware components and/or operating system operations via Javaapplication programs.

Memory module 206 may further include one or more distinct areas (byaddress space and/or other means), which can be utilized by computingdevice 200 to store, among other things, applications and/or other data.For example, one area of memory module 206 may be set aside and employedto store information that describes various capabilities of computingdevice 200, a device identifier, and the like. Such identificationinformation may then be provided to another device based on any of avariety of events, including being sent as part of a header during acommunication, sent upon request, or the like. One common softwareapplication is a browser program that is generally used to send/receiveinformation to/from a web server. In one embodiment, the browserapplication is enabled to employ Handheld Device Markup Language (HDML),Wireless Markup Language (WML), WMLScript, JavaScript, StandardGeneralized Markup Language (SGML), HyperText Markup Language (HTML),eXtensible Markup Language (XML), and the like, to display and send amessage. However, any of a variety of other web based languages may alsobe employed. In one embodiment, using the browser application, a usermay view an article or other content on a web page with one or morehighlighted portions as target objects.

Display interface 214 may be coupled with a display unit (not shown),such as liquid crystal display (LCD), gas plasma, light emitting diode(LED), or any other type of display unit that may be used with computingdevice 200. Display units coupled with display interface 214 may alsoinclude a touch sensitive screen arranged to receive input from anobject such as a stylus or a digit from a human hand. Display interface214 may further include interface for other visual status indicators,such Light Emitting Diodes (LED), light arrays, and the like. Displayinterface 214 may include both hardware and software components. Forexample, display interface 214 may include a graphic accelerator forrendering graphic-intensive outputs on the display unit. In oneembodiment, display interface 214 may include software and/or firmwarecomponents that work in conjunction with CPU 204 to render graphicoutput on the display unit.

Audio interface 216 is arranged to produce and receive audio signalssuch as the sound of a human voice. For example, audio interface 216 maybe coupled to a speaker and microphone (not shown) to enablecommunication with a human operator, such as spoken commands, and/orgenerate an audio acknowledgement for some action.

Input devices 218 may include a variety of device types arranged toreceive input from a user, such as a keyboard, a keypad, a mouse, atouchpad, a touch-screen (described with respect to display interface214), a microphone for spoken command input (describe with respect toaudio interface 216), and the like.

I/O processor 220 is generally employed to handle transactions andcommunications with peripheral devices such as mass storage, network,input devices, display, and the like, which couple computing device 200with the external world. In small, low power computing devices, such assome mobile devices, functions of the I/O processor 220 may beintegrated with CPU 204 to reduce hardware cost and complexity. In oneembodiment, I/O processor 220 may be the primary software interface withall other device and/or hardware interfaces, such as optical storage202, hard disk 232, interfaces 226-228, display interface 214, audiointerface 216, and input devices 218.

An electrical bus 222 internal to computing device 200 may be used tocouple various other hardware components, such as CPU 204, memory module206, I/O processor 220, and the like, to each other for transferringdata, instructions, status, and other similar information.

Non-volatile memory 224 may include memory built into computing device200, or portable storage medium, such as USB drives that may include PCMarrays, flash memory including NOR and NAND flash, pluggable hard drive,and the like. In one embodiment, portable storage medium may behavesimilarly to a disk drive. In another embodiment, portable storagemedium may present an interface different than a disk drive, forexample, a read-only interface used for loading/supplying data and/orsoftware.

Various other interfaces 226-228 may include other electrical and/oroptical interfaces for connecting to various hardware peripheral devicesand networks, such as IEEE 1394 also known as FireWire, Universal SerialBus (USB), Small Computer Serial Interface (SCSI), parallel printerinterface, Universal Synchronous Asynchronous Receiver Transmitter(USART), Video Graphics Array (VGA), Super VGA (SVGA), and the like.

Network Interface Card (NIC) 230 may include circuitry for couplingcomputing device 200 to one or more networks, and is generallyconstructed for use with one or more communication protocols andtechnologies including, but not limited to, Global System for Mobilecommunication (GSM), code division multiple access (CDMA), time divisionmultiple access (TDMA), user datagram protocol (UDP), transmissioncontrol protocol/Internet protocol (TCP/IP), SMS, general packet radioservice (GPRS), WAP, ultra wide band (UWB), IEEE 802.16 WorldwideInteroperability for Microwave Access (WiMax), SIP/RTP, Bluetooth,Wi-Fi, Zigbee, UMTS, HSDPA, WCDMA, WEDGE, or any of a variety of otherwired and/or wireless communication protocols.

Hard disk 232 is generally used as a mass storage device for computingdevice 200. In one embodiment, hard disk 232 may be a Ferro-magneticstack of one or more disks forming a disk drive embedded in or coupledto computing device 200. In another embodiment, hard drive 232 may beimplemented as a solid-state device configured to behave as a diskdrive, such as a flash-based hard drive. In yet another embodiment, harddrive 232 may be a remote storage accessible over network interface 230or another interface 226, but acting as a local hard drive. Thoseskilled in the art will appreciate that other technologies andconfigurations may be used to present a hard drive interface andfunctionality to computing device 200 without departing from the spiritof the present disclosure.

Power supply 234 provides power to computing device 200. A rechargeableor non-rechargeable battery may be used to provide power. The power mayalso be provided by an external power source, such as an AC adapter or apowered docking cradle that supplements and/or recharges a battery.

Transceiver 236 generally represents transmitter/receiver circuits forwired and/or wireless transmission and receipt of electronic data.Transceiver 236 may be a stand-alone module or be integrated with othermodules, such as NIC 230. Transceiver 236 may be coupled with one ormore antennas for wireless transmission of information.

Antenna 238 is generally used for wireless transmission of information,for example, in conjunction with transceiver 236, NIC 230, and/or GPS242. Antenna 238 may represent one or more different antennas that maybe coupled with different devices and tuned to different carrierfrequencies configured to communicate using corresponding protocolsand/or networks. Antenna 238 may be of various types, such asomni-directional, dipole, slot, helical, and the like.

Haptic interface 240 is configured to provide tactile feedback to a userof computing device 200. For example, the haptic interface may beemployed to vibrate computing device 200, or an input device coupled tocomputing device 200, such as a game controller, in a particular waywhen an event occurs, such as hitting an object with a car in a videogame.

Global Positioning System (GPS) unit 242 can determine the physicalcoordinates of computing device 200 on the surface of the Earth, whichtypically outputs a location as latitude and longitude values. GPS unit242 can also employ other geo-positioning mechanisms, including, but notlimited to, triangulation, assisted GPS (AGPS), E-OTD, CI, SAI, ETA, BSSor the like, to further determine the physical location of computingdevice 200 on the surface of the Earth. It is understood that underdifferent conditions, GPS unit 242 can determine a physical locationwithin millimeters for computing device 200. In other cases, thedetermined physical location may be less precise, such as within a meteror significantly greater distances. In one embodiment, however, a mobiledevice represented by computing device 200 may, through othercomponents, provide other information that may be employed to determinea physical location of the device, including for example, a MAC address,IP address, or the like.

The above-described illustrative operating environment and computingdevice configuration generally contemplate embodiments in which acomputing device is used in a client/server network communicationenvironment. However, in other embodiments, devices implementingmemory/storage described herein may be used in other networkcommunication environments (e.g., a peer-to-peer network) or even innon-networked environments (e.g., as a non-networked digital camera,music player, etc.).

FIG. 3 shows a memory array 300 of storage cells. In one embodiment, thememory array 300 includes row address line 304 and column address line306 used to select one bit of information stored in storage cell 302.Memory array 300 may be a planar array, a linear array, or an arrayorganized in a different physical arrangement than a two-dimensional(2-D) array. In another embodiment, array 300 may be arranged as a 3-Darray each cell 302 of which is addressable by three independent addresslines (not shown). In yet another embodiment, array 300 may be anN-dimensional array each storage cell of which is addressed using Ndistinct address components. For example, in a 4-D array, one addresscomponent may identify a memory bank; another may identify a memorysegment, while other address components may identify a row and a column.Those skilled in the art will appreciate that many memory arraystructures are possible without departing from the spirit of thedisclosure.

In one embodiment, memory array 300 may be coupled with CPU 204 of FIG.2 that generates an address to access a particular storage cell withinarray 300. Memory array 300 may be coupled with CPU 204 indirectly, forexample, using a memory controller for generating address data. In oneembodiment, storage cell 302 may be flash storage cell, as more fullydescribed with respect to FIG. 4 below.

FIG. 4 shows an illustrative embodiment of a flash storage cell that maybe used as a storage element 302 in array 300 of FIG. 3. In oneembodiment, the structure of flash storage cell 302 includes a rowelectrode implemented, for example, by row address line 304, and acolumn electrode implemented, for example, by column address line 306.Flash storage cell 302 may include control gate 308, insulator 314surrounding floating gate 316, drain 310, and source 312.

Line 304 may be coupled to control gate 308 to control the operation ofthe storage cell. Line 306 may be coupled with drain 310 to controlread/write operation to individual storage cells within array 300 ofFIG. 3. Drain 310, source 312, and floating gate 316 together form atransistor. When this transistor is turned on by voltages applied tolines 304 and 306, a fraction of the charge conducted through thetransistor is accumulated at the insulator 314, via a hot electroninjection process (used, for example, in NOR flash memory), and staysthere until discharged. Generally, the charge that accumulates on theinsulator 314 provides the non-volatility feature of the flash memory.By applying appropriate voltages to lines 304 and 306, the accumulatedcharge on insulator 314 may be removed, for example, via a tunnelingprocess. Removal of electrons may not be well controlled resulting in acomplete erasure of the storage cell. Thus, many non-volatile memorytechnologies, such as flash memory, which depend on charge accumulationand maintenance to hold data in absence of power, use a unidirectionalprocess of writing data in the sense that charge may be added to thecell in a controlled and gradual manner (direction of charge), butcannot be removed gradually (direction of discharge) with adequateprecision or at all.

In one embodiment, each cell of a flash memory may be used to store oneor more bits of discrete information by providing multiple charge levelsthat correspond to multiple states or cell levels. For example, a cellwith four states may be used to represent two bits of information, eachlevel corresponding to one combination of two binary bits, as more fullydescribed below with respect to FIG. 6.

FIG. 5 shows a graph depicting an illustrative example of data and ECCcheck bits. Generally, data stored in memory, volatile and non-volatilealike may get corrupted for various reasons, such as electrical noise,cosmic rays, momentary voltage fluctuations/spikes in a computer system,and the like. Similarly, data may get corrupted during transmission forsimilar reasons. Corruption of binary data generally takes the form offlipped bits. That is, a bit that is originally binary 1 turns to binary0, and vice versa. Many techniques are available to detect and/correctbit errors. For example, bit repetition schemes, parity schemes,checksums, CRC (Cyclic Redundancy Check), Hamming code based schemes,and the like.

The general idea behind error detection and correction schemes is thatsome extra check bits are calculated based on an algorithm from originaluser data bits, which are stored and/or transmitted with the originaldata bits. If an error occurs during storage or transmission, the checkbits are used to detect and/or correct such bit errors. How many biterrors may be detected or corrected is generally dependent on thedetection/correction scheme used as well as the number of check bits.Generally, the more check bits that are used the more bit errors may bedetected and/or corrected. Such error protection increases the cost ofstorage and/or transmission by increasing the amount of storage and/ortransmission bandwidth needed to store and/or transmit the extra checkbits. The original inputs bits (i.e., the data before application oferror detection/correction scheme) and the check bits together form acodeword. Thus, a codeword may be partitioned into two distinctportions: output data representing the input data, and check bitsgenerated as a result of applying the error detection/correction schemeto the input data.

There are two broad and main categories of error detection/correctionschemes: block and convolutional codes. Block codes include schemes thatoperate on fixed-sized blocks or pages of data, for example, data blocksof 8 bits, 64 bits, 512 bits, and the like. Convolutional schemesoperate on streams of bits of arbitrary length without definite orpredefined boundaries. Block and convolutional schemes may also becombined into more comprehensive and complicated concatenated codingschemes for error detection/correction.

ECC is a common scheme used for binary error detection/correction. Theillustrative example of FIG. 5 shows an ECC codeword 500 includingoutput data word 502 and check word 504. ECC code may be represented asa 3-tuple [n, k, d], where n is the total length of codeword 500, k isthe length of the output data, and d is the minimum distance (forexample, Hamming distance). A code may correct t bits of corrupted datawhen the minimum distance, d=2t+1. The number of check bits, m=n−k. Inthe example shown, [n, k, d]=[16, 8, 5], indicating that total codeword500 has 16 bits, 8 of which form the output data. The minimum distancefor this ECC code, d=5. As data stored in a storage cell are representedby a particular coding of the cell levels, such coding can affect theapplication of ECC to the (input) data.

FIG. 6 shows an illustrative flash storage cell coding scheme 600. Asnoted above, cell coding schemes map a binary number to a cell level orstate. An n-level storage cell, including levels L0, L1, . . . , Ln−1,may be mapped to n=2^(x) binary numbers, where x is an integer. Thebinary numbers need not be in numerical order. For example, the 4-levelstorage cells with cell levels 602 may be mapped to the binary sequence604. As shown, the mapping maps L0=11, L1=10, L2=00, and L3=01. Thiscell coding scheme may be referred to as a Gray code. In one embodiment,a 4-level storage cell may be used to represent 2 bits of data accordingto the binary mapping/coding described above. For example, to representan 8-bit data word, four storage cells each having 4 levels L0-L3 may beused, where any possible combination of two bits in the 8-bit data wordmay be represented by one of the L0-L3 cell levels of the correspondingstorage cell. A data word, such as an 8-bit or 16-bit word, may bestored in groups of four or eight storage cells, respectively, each cellhaving four cell levels to represent two bits of the 8-bit or 16-bitword. Such storage cell groups are generally addressed as one collectiveunit from a user or computer software perspective. For example, a groupof four cells holding a byte (an 8-bit word) of data may be arrangedsuch that the data contained in the group may be read or written using asingle address reference. Those skilled in the art will appreciate thatother length words and/or other number of cell levels may be used tostore data.

Different cell level codings may affect the way ECC detects and/orcorrects bit errors. In unidirectional memories, such as NOR and NANDflash memories, writing or charging the storage cell proceeds in onedirection from an initial level (for example, L0, in a 4-level cell) toa final level (for example, L3, in a 4-level cell). In suchunidirectional memories, three distinct data categories may beidentified: an initial category where all cells have the same value,such as binary 1, an arbitrary category where cells have arbitrarybinary values (for example, representing random or arbitrary user data),and a final data category where all cells have the same value which iscomplementary to the value of the initial category, such as binary 0.The initial data category may correspond to a storage cell state wherethe accumulated charge in the cell is at a lowest point (for example,L0). The arbitrary data category may correspond to a storage cell statewhere the accumulated charge in the cell is at an intermediate level(for example, L1 or L2). And the final data category may correspond to amemory state where the accumulated charge in the cell is at a highestlevel (for example, L3).

Generally, cell levels that represent user data may transition twice:once from the initial cell state representing the initial data category(for example, all 1's) to an arbitrary, or intermediate, cell staterepresenting the arbitrary data category, and once from the arbitrarycell state to a final cell state representing the final data category(for example, all 0's). Now, to protect user data with coherent checkbits that allow detection and correction of t error bits within k userdata bits, all three data categories described above may need to becovered with coherent check bits. In one embodiment, coverage of thedata categories by coherent check bits may be done by adding more checkbits, using additional storage cells beyond those already being used torepresent the check bits, at the expense of additional storage and/ortransmission bandwidth requirements. In another embodiment, the datacategories may be covered by transforming a bit stream representing userdata to a bit stream representing internal data, which may then bewritten to the cells according to the selected cell coding scheme,possibly in addition to employing a check bit inversion technique, asmore fully described below with respect to FIGS. 7-9. Using theappropriate transformation and check bit inversions, the coherence ofthe check bits may be maintained as the storage cell transitions fromthe arbitrary state to the final state, during the second write, withouterasing the storage cell or adding additional check bits.

In order to provide a coherent check word for a first data word, anintermediate data word, and a final data word, without using additionalcheck bits, the internal cell coding scheme, transformation, and checkbit inversions may be designed so that two conditions are satisfied. Thefirst condition is that all of the cell states of the storage cellscorresponding to the coherent check bits are at a final state when allof the cell states of the storage cells corresponding to the data bitsare at the final state. The second condition is that all of the cellstates of the storage cells corresponding to the coherent check bits areat the initial state when all of the cell states of the storage cellscorresponding to the data bits are at the initial state.

FIG. 7 shows an illustrative matrix A used in calculation of ECC. ECCmay be applied to data by employing a generator matrix G=[I|A], where Iis the k×k identity matrix (same dimension as the k data bits beingprotected by ECC), and A 702 is a [k×[n−k]] matrix (having columns 704by rows 706) that defines the mathematical transformation for generatingthe check bits. Matrix G is a [k×n] matrix formed from the concatenationof matrices I and A. Multiplying data by G using a cross-product matrixoperation generates a codeword similar in structure to codeword 500 ofFIG. 5, where the k data bits are preserved in the first k bits of then-bit codeword, followed by the [n−k] check bits.

FIG. 8 shows an illustrative application of matrix A to user data.Applying matrix A, without using matrix I, to the data generates thecheck bits portion of the codeword. Two sets of data 802 and 804 areshown to illustrate the operation of matrix A and check bit inversion.The changes to input data set 802 and 804 in this example may befollowed independently and in parallel. Starting with data set 802, thedata bits 0101 0101 in bit positions 0-7 are coded according to the Graycoding 604 of FIG. 6, which may result in the storage cells being set tothe highest or final level, e.g., L3. The bit stream 0101 0101 may beinternal data that results from a transformation of user data bit streamof 0000 0000 as will described with reference to FIG. 9. As the databits of data set 802 are cross multiplied by matrix A, check word 806with corresponding check bits in bit positions 0-7 is generated.

Check word 806, however, does not satisfy the above-identified conditionthat all of the parity cells are at a final state when all of the datacells are at a final state. Accordingly, selected bits, e.g., bits 0 and4-6, of check word 806 may be inverted to generate check word 812.Inverting bits of check word 806 uses the same storage cell representingthe existing check word 806, in contrast to using additional memory torepresent a new check word, thus not increasing memory usage. In oneembodiment, such bit inversion may be performed using inverters 810.Those skilled in the art will appreciate that bit inversion may beperformed in hardware, software, or a combination of both depending onapplication, flexibility, and performance needs.

Similarly, data set 804, which may be represented by data cells in aninitial state L0 when written according to a Gray code, may betransformed by matrix A to check bits 808 and inverted using theidentified inversion bit positions (for example, 0 and 4-6) to generatecheck word 814. Check word 814 may satisfy the other conditionidentified above, e.g., that all of the parity cells are at an initialstate when all of the data cells are at an initial state.

Thus, using transformation, internal coding, and bit conversion, asdescribed, allows the generation of coherent check words for an initial,intermediate, and final data word.

FIG. 9 shows an illustrative application of transformation and cellcoding. In the example shown, an initial user data word 904 includesdata bits at positions 0-7=1111 1111. The user data word 904 may betransformed to internal data word 910 with data bits 1111 1111 accordingto the transformation function shown in the left two columns of table902. An ECC may generate a check word 916 based on the internal dataword 910 and selected bit inversion described above. The right twocolumns of table 902 represent the internal coding scheme that may beused for the coding of the internal data word 910 and the check word 916into respective data cells and check cells.

The initial user data word 904 may be transitioned to an arbitrary userdata word 906, e.g., 1000 1101. The user data word 906 may be internallyrepresented by internal data word 912, e.g., 1001 1100 as transformed bythe transformation function of table 902. Check word 918 may begenerated based on internal data word 912 as described above.

The user data word 906 may again be transitioned to a final user dataword 908, e.g., 0000 0000. The user data word 908 may be internallyrepresented by internal data word 914, e.g., 0101 0101 as transformed bythe transformation function of table 902. Check word 920 may begenerated based on internal data word 914 as described above.

The check words, e.g., check words 916, 918, and 920 may be coherentwith the check cells satisfying the above-identified conditions.

The initial user data words 904, 906, 908, may be consecutively receivedso that the generating and writing of the internal data words 910, 910,and 914 and check words 916, 918, and 920 may also be consecutivelygenerated and written. Thus, there will be no intervening eraseoperations between these consecutive operations.

FIG. 10 shows an illustrative process of protecting user data with ECCcheck bits according to aspects of present disclosure. At block 1010,the process may transform a bit stream of user data, e.g., a user dataword, to a bit stream of internal data, e.g., an internal data word,according to a selected transformation function. The user data word maybe the same size as the internal data word. The transformation functionused may be similar to the transformation function described in table902.

At block 1020, the process may generate one or more parity bits, e.g., acheck word, with selected bits inverted, according to an ECC. Thegeneration of the check word may be based at least in part on a functionof the internal data word and a matrix of the ECC. Selection of the bitsfor inversion may be based at least in part on satisfying the twoconditions identified above, e.g., that the check cell states equal tothe data cell states when the data cell states are either in an initialstate or a final state.

At block 1030, the process may write the internal data word and thecheck word to corresponding storage cells. This may be done, e.g.,according to an internal cell coding scheme such as the one identifiedabove in FIG. 6.

The process described in FIG. 10 may be reversed to recover the internaldata word, the check word, and, ultimately, the user data.

It will be understood that each block of the flowchart illustration, andcombinations of blocks in the flowchart illustration, can be implementedby computer program instructions. These program instructions may beprovided to a processor to produce a machine, such that theinstructions, which execute on the processor, create means forimplementing the actions specified in the flowchart block or blocks. Thecomputer program instructions may be executed by a processor to cause aseries of operations to be performed by the processor to produce acomputer implemented process such that the instructions, which executeon the processor to provide operations for implementing the actionsspecified in the flowchart block or blocks. The computer programinstructions may also cause at least some of the operations shown in theblocks of the flowchart to be performed in parallel. Moreover, some ofthe operations may also be performed across more than one processor,such as might arise in a multi-processor computer system. In addition,one or more blocks or combinations of blocks in the flowchartillustration may also be performed concurrently with other blocks orcombinations of blocks, or even in a different sequence than illustratedwithout departing from the scope or spirit of the invention.

Accordingly, blocks of the flowchart illustration support combinationsof means for performing the specified actions, combinations ofoperations for performing the specified actions and program instructionmeans for performing the specified actions. It will also be understoodthat each block of the flowchart illustration, and combinations ofblocks in the flowchart illustration, can be implemented by specialpurpose hardware-based systems which perform the specified actions oroperations, or combinations of special purpose hardware and computerinstructions.

Changes can be made to the claimed invention in light of the aboveDetailed Description. While the above description details certainembodiments of the invention and describes the best mode contemplated,no matter how detailed the above appears in text, the claimed inventioncan be practiced in many ways. Details of the system may varyconsiderably in its implementation details, while still beingencompassed by the claimed invention disclosed herein.

Particular terminology used when describing certain features or aspectsof the invention should not be taken to imply that the terminology isbeing redefined herein to be restricted to any specific characteristics,features, or aspects of the invention with which that terminology isassociated. In general, the terms used in the following claims shouldnot be construed to limit the claimed invention to the specificembodiments disclosed in the specification, unless the above DetailedDescription section explicitly defines such terms. Accordingly, theactual scope of the claimed invention encompasses not only the disclosedembodiments, but also all equivalent ways of practicing or implementingthe claimed invention.

The above specification, examples, and data provide a completedescription of the manufacture and use of the composition of theinvention. Since many embodiments of the invention can be made withoutdeparting from the spirit and scope of the invention, the inventionresides in the claims hereinafter appended.

1. A method comprising: receiving a user data word; transforming theuser data word to an internal data word according to a transformationfunction, the user data word being the same size as the internal dataword; writing the internal data word to a plurality of unidirectionaldata storage cells according to an internal cell coding scheme; andgenerating a check word based at least in part on the internal data wordaccording to an error correction code (ECC).
 2. The method of claim 1,wherein said generating the check word comprises: generating anintermediate check word based at least in part on an ECC matrix and theinternal data word; and inverting one or more bits of the intermediatecheck word.
 3. The method of claim 1, further comprising: writing thecheck word to at least one unidirectional data storage cell according tothe internal cell coding scheme.
 4. The method of claim 3, wherein thetransformation function, the internal cell coding scheme, and the ECC,are such that a state of all of the least one unidirectional datastorage cells match a state of all of the plurality of unidirectionaldata storage cells when all of the plurality of unidirectional datastorage cells are either in an initial state or a final state.
 5. Themethod of claim 1, wherein the internal user data word is an initialuser data word, the internal data word is an initial internal data wordand the check word is an initial check word and the method furthercomprises: receiving a plurality of user data words, to update the userdata word; transforming the plurality of user data words to a respectiveplurality of internal data words; writing the plurality of internal datawords to the plurality of unidirectional data storage cells; andgenerating a plurality of check words that respectively correspond tothe plurality of internal data words, wherein said writing of theplurality of internal data words and generating the plurality of checkwords occurs without erasing the plurality of unidirectional datastorage cells.
 6. An apparatus comprising: a plurality of unidirectionaldata storage cells; and a controller coupled to the plurality ofunidirectional data storage cells, and configured to: consecutivelywrite at least three data words into one or more of the plurality ofunidirectional data storage cells; generate a check word thatcorresponds to each of the at least three data words; and consecutivelywrite the at least three check words into at least one of the pluralityof unidirectional data storage cells, wherein the controller isconfigured to consecutively write the at least three data words, togenerate the at least three check words, and to consecutively write theat least three check words such that a state of all of the at least oneunidirectional data storage cell matches a state of all of the one ormore unidirectional data storage cells when all of the one or moreunidirectional data storage cells are either in an initial state or afinal state.
 7. The apparatus of claim 6, wherein the controller isconfigured to: generate an intermediate check word based at least inpart on a data word and an error correction code matrix; and invert oneor more bits of the intermediate check word to generate a check wordthat corresponds to the data word.
 8. The apparatus of claim 6, whereinthe at least three data words includes a first internal data word andthe controller is configured to: receive a first user data word; andtransform the first user data word to the first internal data wordaccording to a transformation function, the first user data word beingthe same size as the first internal data word.
 9. The apparatus of claim6, wherein the controller is configured to write the at least three datawords and the at least three check words according to an internal cellcoding scheme.
 10. The apparatus of claim 6, wherein the controller isconfigured to: write a first data word of the at least three data wordsby setting the one or more unidirectional data storage cells to aninitial state; write a first check word of the at least three checkwords by setting the at least one unidirectional data storage cell to aninitial state; write a second data word of the at least three data wordsby setting the one or more unidirectional data storage cells to a finalstate; and write a second check word of the at least three check wordsby setting the at least one unidirectional data storage cell to a finalstate.
 11. A method comprising: consecutively writing at least threedata words into one or more unidirectional data storage cells;generating a check word that corresponds to each of the at least threedata words; and consecutively writing the at least three check wordsinto at least one unidirectional data storage cell, wherein theconsecutively writing of the at least three data words, the generatingof the at least three check words, and the consecutively writing of theat least three check words is such that a state of all of the at leastone unidirectional data storage cell matches a state of all of the oneor more unidirectional data storage cells when all of the one or moreunidirectional data storage cells are either in an initial state or afinal state.
 12. The method of claim 11, further comprising: generatingan intermediate check word based at least in part on a data word and anerror correction code matrix; and inverting one or more bits of theintermediate check word to generate a check word that corresponds to thedata word.
 13. The method of claim 11, wherein the at least three datawords includes a first internal data word and the method furthercomprises: receiving a first user data word; and transforming the firstuser data word to the first internal data word according to atransformation function, the first user data word being the same size asthe first internal data word.
 14. The method of claim 11, wherein saidconsecutively writing of the at least three data words and the at leastthree check words is done according to an internal cell coding scheme.15. The method of claim 11, further comprising: writing a first dataword of the at least three data words by setting the one or moreunidirectional data storage cells to an initial state; writing a firstcheck word of the at least three check words by setting the at least oneunidirectional data storage cell to an initial state; writing a seconddata word of the at least three data words by setting the one or moreunidirectional data storage cells to a final state; and writing a secondcheck word of the at least three check words by setting the at least oneunidirectional data storage cell to a final state.
 16. A machinereadable medium having associated instructions that, when executed,cause the machine to: receive a user data word; transform the user dataword to an internal data word according to a transformation function,the user data word being the same size as the internal data word; writethe internal data word to a plurality of unidirectional data storagecells according to an internal cell coding scheme; and generate a checkword based at least in part on the internal data word according to anerror correction code (ECC).
 17. The machine readable medium of claim16, wherein the associated instructions, when executed, cause themachine to: generate an intermediate check word based at least in parton an ECC matrix and the internal data word; and invert one or more bitsof the intermediate check word to generate the check word.
 18. Themachine readable medium of claim 17, wherein the associatedinstructions, when executed, cause the machine to: write the check wordto at least one unidirectional data storage cell according to theinternal cell coding scheme.
 19. The machine readable medium of claim16, wherein the transformation function, the internal cell codingscheme, and the ECC, are such that a state of all of the least oneunidirectional data storage cells match a state of all of the pluralityof unidirectional data storage cells when all of the plurality ofunidirectional data storage cells are either in an initial state or afinal state.
 20. The machine readable medium of claim 16, wherein theinternal user data word is an initial user data word, the internal dataword is an initial internal data word and the check word is an initialcheck word and the associated instructions, when executed, cause themachine to: receive a plurality of user data words, to update the userdata word; transform the plurality of user data words to a respectiveplurality of internal data words; write the plurality of internal datawords to the plurality of unidirectional data storage cells; andgenerate a plurality of check words that respectively correspond to theplurality of internal data words, wherein said writing of the pluralityof internal data words and generating the plurality of check wordsoccurs without erasing the plurality of unidirectional data storagecells.